From 452c1b60120aee0883c3339b363f820b8d69c299 Mon Sep 17 00:00:00 2001
From: Vladimir Marko <vmarko@google.com>
Date: Fri, 25 Sep 2015 14:44:17 +0100
Subject: Optimizing: Simplify UShr+And, Shr+And.

Eliminate And from UShr+And if the And-mask contains all the
bits that can be non-zero after UShr. Transform Shr+And to
UShr if the And-mask precisely clears the shifted-in sign
bits.

This prepares for detecting the Rotate pattern, i.e.
  (x << N) | (x >>> (SIZE - N))
in code that unnecessarily masks the UShr, for example
  (x << 1) | ((x >>> 31) & 1) ,
or uses Shr, for example
  (x << 8) | ((x >> 24) & 0xff) .

Change-Id: I684c4b752547d9b1057d0d4c4d44550bb1a3ffb4
---
 compiler/optimizing/instruction_simplifier.cc | 47 ++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 8 deletions(-)

(limited to 'compiler/optimizing')

diff --git a/compiler/optimizing/instruction_simplifier.cc b/compiler/optimizing/instruction_simplifier.cc
index 0ac26de674..abdda13bb8 100644
--- a/compiler/optimizing/instruction_simplifier.cc
+++ b/compiler/optimizing/instruction_simplifier.cc
@@ -502,14 +502,45 @@ void InstructionSimplifierVisitor::VisitAnd(HAnd* instruction) {
   HConstant* input_cst = instruction->GetConstantRight();
   HInstruction* input_other = instruction->GetLeastConstantLeft();
 
-  if ((input_cst != nullptr) && AreAllBitsSet(input_cst)) {
-    // Replace code looking like
-    //    AND dst, src, 0xFFF...FF
-    // with
-    //    src
-    instruction->ReplaceWith(input_other);
-    instruction->GetBlock()->RemoveInstruction(instruction);
-    return;
+  if (input_cst != nullptr) {
+    int64_t value = Int64FromConstant(input_cst);
+    if (value == -1) {
+      // Replace code looking like
+      //    AND dst, src, 0xFFF...FF
+      // with
+      //    src
+      instruction->ReplaceWith(input_other);
+      instruction->GetBlock()->RemoveInstruction(instruction);
+      RecordSimplification();
+      return;
+    }
+    // Eliminate And from UShr+And if the And-mask contains all the bits that
+    // can be non-zero after UShr. Transform Shr+And to UShr if the And-mask
+    // precisely clears the shifted-in sign bits.
+    if ((input_other->IsUShr() || input_other->IsShr()) && input_other->InputAt(1)->IsConstant()) {
+      size_t reg_bits = (instruction->GetResultType() == Primitive::kPrimLong) ? 64 : 32;
+      size_t shift = Int64FromConstant(input_other->InputAt(1)->AsConstant()) & (reg_bits - 1);
+      size_t num_tail_bits_set = CTZ(value + 1);
+      if ((num_tail_bits_set >= reg_bits - shift) && input_other->IsUShr()) {
+        // This AND clears only bits known to be clear, for example "(x >>> 24) & 0xff".
+        instruction->ReplaceWith(input_other);
+        instruction->GetBlock()->RemoveInstruction(instruction);
+        RecordSimplification();
+        return;
+      }  else if ((num_tail_bits_set == reg_bits - shift) && IsPowerOfTwo(value + 1) &&
+          input_other->HasOnlyOneNonEnvironmentUse()) {
+        DCHECK(input_other->IsShr());  // For UShr, we would have taken the branch above.
+        // Replace SHR+AND with USHR, for example "(x >> 24) & 0xff" -> "x >>> 24".
+        HUShr* ushr = new (GetGraph()->GetArena()) HUShr(instruction->GetType(),
+                                                         input_other->InputAt(0),
+                                                         input_other->InputAt(1),
+                                                         input_other->GetDexPc());
+        instruction->GetBlock()->ReplaceAndRemoveInstructionWith(instruction, ushr);
+        input_other->GetBlock()->RemoveInstruction(input_other);
+        RecordSimplification();
+        return;
+      }
+    }
   }
 
   // We assume that GVN has run before, so we only perform a pointer comparison.
-- 
cgit v1.2.3-59-g8ed1b